Stinging caterpillars are invading Paris only weeks after a panic over bedbugs in the French capital.

Pine processionary caterpillars – whose name refers to how they travel in single file – have arrived three months earlier than usual because of unusually mild temperatures, experts say.

Entire neighbourhoods have reportedly been infested by the insects, which measure up to 4cm (1.6in) in length, live in groups and weave silk nests in colonised pine and oak trees.

While the caterpillars do not bite, their hairs are like “microscopic needles or harpoons” that detach from their bodies when they feel threatened and travel with the wind, Anses, the French health and safety agency, said.

The hairs contain a toxic protein that can cause blisters on the skin and a variety of allergic reactions. The caterpillars and their hairs can also be fatal for pets and toddlers who may inadvertently put them in their mouths.

In 2022, processionary caterpillars were designated by the government as harmful to human health.

After finding huge colonies nesting in his 15 metre-high pine tree, one homeowner in the northern department of Yveline was forced to cut down the tree, which he had planted 20 years ago.

“I couldn’t believe it,” Emmanuel told Le Parisien. “They came down from my pine despite the trap I set. They are devouring my tree and I am afraid for my dog.”

In the final larval stages, pine processionary caterpillars descend along the trunk to bury themselves in the ground and transform into a chrysalis.

“It’s an invasion,” Jean-Marc Pommier, the mayor of Bonnieres-sur-Seine, told Le Parisien. “People are calling the town hall. We have entire neighbourhoods affected, we see trees covered with these horrible garlands.”

The caterpillar invasion follows a major Paris bedbug scare that made locals and tourists skittish about travelling through the city.